In recent years, there has been a rapid progression to the use of optical fiber access networks, and video distribution services using these mass access networks are steadily gaining popularity. Due to the increase in demand for high-capacity-data communication in both hardware and software, the demand for higher bit rates in optic fiber communication is increasing.
However, various problems have surfaced in attempts to increase the bit rate of optic fiber communications. Firstly, along with an improvement in the bit rate of optic fiber communications, it is necessary to increase the operating speed of the electrical devices and optical devices used in transceivers. At present, in the IM-DD method (Intensity Modulation-Direct Detection) which is mainly used for optic fiber communications, there is a simple technique wherein, in the transmitter, “0”, “1” of an electrical signal are replaced by OFF, ON of a light, and in the receiver, “0”, “1” of the electrical signal is then reproduced again. Therefore, if it is attempted to improve 10 Gbps which is the current norm to 40 Gbps, it is necessary to increase the operating speed of optical devices such as lasers and photodiodes, or electrical devices such as electrical amplifiers which drive them as well as their discrimination circuits, by 4 times. In addition to the technical problems involved in operating electrical and optical devices at 40 Gbps, there is also the problem of a high material and manufacturing cost.
When bit rates used in optic fiber communications are increased, the light waveform will deteriorate due to the chromatic dispersion in the optical fiber, and the transmission rate and transmission distance will thereby be limited. Chromatic dispersion (hereafter, distribution) is the wavelength dependency of the group velocity with which a signal spreads in an optical fiber.
Strictly speaking, a light waveform has plural wavelength components (i.e., spectral broadening), and if the group velocity has wavelength dependency, there will be a component which propagates slowly and a component which propagates rapidly in the optical fiber, and as a result, the waveform will broaden. Hence, if the amount of dispersion cannot be ignored, waveform distortion will occur and receiving characteristics will deteriorate.
Since the amount of dispersion is proportional to the length of the fiber, the result is that the transmission distance is limited. The transmission distance of a chromatic dispersion limit is in inverse proportion to the square of the transmission bandwidth. For example, if it is attempted to convert a signal propagating at 10 Gbps to 40 Gbps, the distance will be reduced to 1/16.
When bit rates used in optic fiber communication are increased, the light waveform will deteriorate due also to the polarization dispersion in the optical fiber, and this also limits the transmission rate and transmission distance.
Regarding polarization dispersion, due to physical stress, or environmental factors such as temperature and humidity, the perfect circularity of cross-section with which the optical fiber was designed will deteriorate slightly, and this will lead to propagation in two modes even in a single mode fiber.
Since the propagation speed differs slightly between modes, waveform broadening will again occur and distance will be limited. The transmission distance at the polarization dispersion limit is in inverse proportion to the transmission bandwidth, and as in the previous example, if it is attempted to convert a signal propagating at 10 Gbps to 40 Gbps, the distance will be reduced to ¼.
The increase of bit rate also brings about an increase in signal bandwidth occupancy. For example, it is attempted to increase the bit rate by 4 times, the signal bandwidth occupancy, i.e., the spectral bandwidth of the occupying signal, is also increased by 4 times. If it is attempted to convert an optical signal to multi-channel in the wavelength direction such as in wavelength multiplexed transmission (WDM), the bandwidth is then limited by the amplification bandwidth of the optical amplifier which performs package amplification of this wavelength multiplexing signal. Specifically, if it is desired to set wavelengths so that wavelength multiplexing signals do not overlap, the product of the occupied spectral bandwidth and the wavelength number must necessarily be the amplification bandwidth of this optical amplifier. Since the amplification bandwidth is constant and it is necessary to decrease the wavelength number if the occupied spectral bandwidth is increased, even if the bit rate is improved, the spectral bandwidth will increase by a corresponding amount and the wavelength number will decrease, so the total capacity does not change and conversion to high capacity will thus be limited.
Optical multi-level modulation is now attracting attention as a way of overcoming the limitations of device response speed, the limitations due to chromatic dispersion and polarization, and the limitations due to broadening of spectral bandwidth. Optical multi-level modulation is a technique whereby, by performing M (M>2) modulations of light intensity, optical phase, or both, the total transmission capacity is increased by log M times (the base of the logarithm is 2), without increasing the bit rate of the modulation drive signal.
Specifically, consider the case where it is desired to form a 40 Gbps signal. With conventional binary transmission, a drive signal of 40 Gbps is required.
In quaternary transmission, on the other hand, the transmission capacity can be increased by log 4=2 times, hence by providing two driving signals of 20 Gbps, 40 Gbps can be achieved. Likewise, in octal transmission, the transmission capacity can be increased by 3 times by providing three signals of approximately 13 Gbps. In 16 value (hexadecimal) transmission, the transmission capacity can be increased by 4 times by providing four signals of 10 Gbps. In both of these cases, therefore, 40 Gbps transmission can be realized.
In transmission using such an optical multi-level modulation signal, the chromatic dispersion, polarization dispersion and occupied spectral bandwidth are limited by the rate of these driving signals, so in the case of a 40 Gbps signal formed by two 20 Gbps signals mentioned hereinabove, as compared to transmitting the 40 GB/s signal by binary transmission according to the prior art, the chromatic dispersion limiting distance can be extended by 4 times, the polarization dispersion limiting distance can be extended by 2 times, and the occupied spectral bandwidth can be reduced to ½.
In optical multi-level modulation, QPSK (Quaternary Phase Shift Keying) wherein the phase of the light is modulated by 4 values, is attracting attention due to the ease of uniformly controlling the gaps between each level, and the improvement of sensitivity due to phase modulation.
As a method of forming a quaternary phase modulation signal, the technique disclosed in JP-T-2004-516743 is often used. The principle will be described using FIG. 1. A signal light outputted from a light source (1) is split in a branching filter (2). The signal light split into two parts reaches phase modulators A and B (3A, 3B), respectively. An electrical signal wherein a bias voltage 1A (6A) is superposed on a data signal A (7A) by a bias superposer (8A), is applied to the phase modulator A. The lightwave signal inputted into the phase modulator A (3A) is subjected to binary phase modulation by this electrical signal, and the result is outputted.
This situation will be described using FIG. 2 and FIG. 3. A Mach-Zehnder (MZ) type interferometer is generally used for the phase modulator. The output characteristic (extinction characteristic) of the MZ modulator is shown in FIG. 2. When the applied voltage (horizontal axis) of the MZ modulator is varied, the output (vertical axis) from the modulator traces a locus which resembles a sinusoidal wave as shown in FIG. 2. The voltage required for the extinction to change from a valley to a peak is a key parameter which is defined as Vπ, and represents the characteristic of the modulator.
As shown in FIG. 3, here, the case will be considered where a binary electrical waveform of amplitude 2Vπ centered on the valley of the extinction characteristic (wherein the bias voltage is made to coincide with the valley of the extinction characteristic), is applied to this modulator. Since modulation is performed from peak to peak of the extinction characteristic, the output of the modulator is a waveform which first changes from a peak to a valley, and then becomes a peak again. Specifically, at the center of a bit, the output is always at the position of a peak and the amplitude is fixed. However, regarding the phase of the light, care is required. At adjacent peaks of the extinction characteristic of a MZ modulator, the phases of the outputted light differ by π. Specifically, when these characteristics are taken into consideration, “0” of the input electrical waveform is converted to an output light of amplitude “1”, phase “0”, and “1” of the input electrical waveform is converted to an output light of amplitude “1”, phase “π”. Specifically, a binary-phase-modulation signal having a fixed amplitude and a phase of “0”, “π” is formed.
FIG. 4 shows the phase states at a point A in FIG. 1, i.e., in the output of the phase modulator A (3A). The graphs of FIG. 4 are diagrams which represent the phase in a complex plane using the I-axis and the Q-axis. The I-axis represents the amount of an in-phase component, and the Q-axis represents the amount of a quadrature component. When arbitrary signal points are disposed on the coordinate axes, the distance of the signal point from the origin represents the amplitude of the signal. The angle between a line which connects the signal point from the origin, and a line which connects the positive direction of the I-axis from the origin, represents the phase of the signal. Thus, for the phase states at the point A, two points are disposed on the I-axis symmetrically around the origin. Specifically, data signals of amplitudes “0”, “1” are changed into two points of phase “0” and phase “π”, respectively.
Likewise, an electrical signal wherein a bias voltage 1B (6B) is superposed on a data signal B (7B) by a bias superposer (8B), is applied to the phase modulator B. The lightwave signal inputted into the phase modulator B (3B) is subjected to binary phase modulation by this electrical signal, and the result is outputted. Thus, a point B in FIG. 1, i.e., the phase state in the output of the phase modulator B (3B), is disposed at two points on the I-axis symmetrical about the origin as in the case of the point A, as shown in FIG. 4. Specifically, data signals of amplitude “0”, “1” are changed into two points of phase “0” and phase “π”, respectively.
The phase adjuster (4) is installed in the output of the phase modulator B (3B), which is one of the two phase modulators. A bias voltage 2 is applied to the phase adjuster (4). The phase of the lightwave signal inputted into the phase adjuster (4) is adjusted by an amount according to this bias voltage 2, and is outputted. As this phase adjustment, π/2 is an ideal value. As a result of this phase adjustment by π/2, the phase states at a point C and a point D give mutually different results, as shown in FIG. 4. Specifically, the phase state at the point D, as a result of rotating each signal point by π/2, shifts to two points disposed on the Q-axis disposed symmetrically about the origin.
The output of the phase modulator A (3A), and the output of the phase adjuster (4), are multiplexed by the multiplexer (5). The output of the multiplexer (5), i.e., the phase state of the signal points at a point E, is shown in FIG. 4. These signal points correspond to four points shown by small circles with dotted lines in the diagram before multiplexing, and as a result of multiplexing by the multiplexer (5), four signal points are output which are a combined electric field, or geometrically, a combined vector, of these four points. Specifically, “x, y” show the state of data A, and the state of data B, respectively, and the signal point when the data A is “0” and the data B is “1”, i.e., “0, 1”, is disposed at a point in the fourth quadrant of the coordinates.
Likewise, the points “0, 0”, “1, 0”, and “1, 1” are disposed in the first, second, and third quadrants, respectively. Hence, a quaternary phase modulation signal having the four phase levels +π/4, +3π/4, −3π/4 and −π/4, is formed.